Help, Herman (our Hymer) is playing silly buggers again!

Whilst cooking dinner tonight my husband pushed in the dial for the grill and the ceiling light came on Before anyone says it no we have not opened the wine yet He then switched the oven light on and again the ceiling light came on! Any ideas please? The grill switch only turns the light on sometimes but it comes on every time we switch the oven light on. Have we got a short somewhere?